Backlight and liquid crystal display device

ABSTRACT

A side light type backlight includes a light source including a plurality of LEDs, and a light guide plate. One of the end surfaces of the light guide plate is a light incidence surface at which a plurality of R-LEDs, a plurality of G-LEDs and a plurality of B-LEDs are arranged. LEDs satisfy the relationship of: a distribution range of light emitted from G-LEDs&lt;a distribution range of light emitted from R-LEDs, or a distribution range of light emitted from G-LEDs&lt;a distribution range of light emitted from B-LEDs. Also, LEDs are electrically connected to each other.

This is a divisional of application Ser. No. 10/830,780, filed Apr. 23,2004

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a side-light type backlight, including a lightsource which includes LEDs and a light guide plate, and a liquid crystaldisplay device.

2. Description of the Related Art

Backlights used conventionally for a liquid crystal display device canbe classified into a side-light type backlight in which a light sourceis arranged on an end surface side of a light guide plate and ajust-below type backlight in which a light source is arranged just belowa liquid crystal panel. The side-light type backlight has mainly beenused for liquid crystal display devices having a size of about 20 inchesor below, particularly for liquid crystal display devices requiring areduced thickness.

In either system, a cold cathode-ray tube has been used for the lightsource. LEDs are being used, at present, for compact display sizes suchas portable cellular telephones and PDAs because much light power is notrequired and LEDs are suitable for reducing the size and the weight.

The cold cathode-ray tube is predominant in the liquid crystal displaydevices having a greater display size than those of the cellulartelephones and the PDA. However, the problem of protecting theenvironment has been taken seriously and the use of the cold cathode-raytube, which uses mercury, is not desirable. Therefore, various lightsources such as mercury-free florescent tubes, LEDs, and so forth havebeen developed to replace the cold cathode-ray tube. Among them, LEDsare very promising as a light source for next generation devices.

The side light type backlight including the LEDs is described inJapanese Unexamined Patent Publications (Kokai) No. 2001-174816 and No.2002-350846, for example. The former patent reference 1 teaches toarrange red, blue and green rod-like light sources along a periphery ofa light guide plate. The latter patent reference 2 teaches to use fourcolors of LEDs, that is, white, red, blue and green LEDs.

When the LEDs are used as the light source in the side light typebacklight, there may be the case where a plurality of white LEDs arearranged along one or a plurality of sides of a light guide plate andthe case where a plurality of red LEDs (R-LEDs), a plurality of greenLEDs (G-LEDs) and a plurality of blue LEDs (B-LEDs) are arranged alongone or a plurality of sides of the light guide plate to create the whitecolor. The following problems occur at this time.

(a) To secure the amount of light in the light guide plate anduniformity of chromaticity. When a plurality of LEDs are arranged alongone side of the light guide plate, these LEDs are arranged with acertain interval among them. Therefore, regions where light is absentappear at portions close to the light incidence surface and, as thedistance increases, light mixes with light from the neighboring LEDs,and light becomes uniform at a position away from the light incidencesurface.

(b) Drop of light emission efficiency due to heat generation of the LEDsand variation among the LEDs. The LEDs are preferably cooled becauselight emission efficiency drops with heat generation. Light emissionefficiency is different among the R-LEDs, the G-LEDs and the B-LEDsdepending on the temperature, and the R-LEDs are most susceptible to theinfluences of the temperature. Unless a LED is satisfactorily cooled,non-uniformity of the amount of light and non-uniformity of color occur.

(c) To secure reliability. When the LEDs are used at a certaintemperature for a long time, light emission efficiency drops. The dropof light emission efficiency is also different among the R-LEDs, theG-LEDs and the B-LEDs. Therefore, there is the case where non-uniformityof the amount of light and non-uniformity of colors appear after the usefor thousands of hours. There is the possibility that deterioration ofany LEDs among a plurality of LEDs results in non-uniformity of theamount of light and non-uniformity of colors when a plurality of LEDsare used.

(d) To improve efficiency. To obtain the amount of light equivalent tothat of the cold cathode-ray tube by using the LEDs, a large number ofLEDs are necessary. This results in the increase in the cost andconsumed power. Therefore, a plurality of LEDs must be utilizedefficiently.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a backlight that caneliminate color non-uniformity and brightness non-uniformity and canhave a high light-emission efficiency, and a liquid crystal displaydevice using the backlight.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, a light guideplate, at least one of end surfaces of the light guide plate being alight incidence surface at which a plurality of R-LEDs, a plurality ofG-LEDs and a plurality of B-LEDs are arranged, wherein the plurality ofLEDs satisfy the relationship of: a distribution range of light emittedfrom G-LEDs<a distribution range of light emitted from R-LEDs or adistribution range of light emitted from G-LEDs<a distribution range oflight emitted from B-LEDs.

According to this construction, when a plurality of R-LEDs, a pluralityof G-LEDs and a plurality of B-LEDs are used, the number of G-LEDs isthe greatest, to provide the white balance, and the number of B-LEDs isthe smallest. Therefore, the interval between two B-LEDs is extended inthe case of the B-LEDs having the smallest number, and the color doesnot mix unless the positions are away from the light incidence surfaceof the light guide plate (blue brightness does not become uniform).Therefore, the distribution range of light outgoing from the B-LEDs mustbe greater than the distribution range of light outgoing from the G-LEDsand the distribution range of light outgoing from the R-LEDs. However,there may be a case where the numbers of the B-LEDs and the B-LEDs mustsatisfy not only the relationship of: B-LEDs<R-LEDs but alsoB-LEDs>R-LEDs. In such a case, the distribution ranges of light are soset as to satisfy the relationship of: G-LEDs<B-LEDs<R-LEDs.

When the light of one B-LED is made incident into the light guide plateand diverges in the light guide plate, the position at which the lightstarts intersecting light of an adjacent B-LED is farther than thepositions of R-LED and G-LED at which they similarly start intersecting.However, because the distribution range of the B-LEDs is wider than thedistribution ranges of the R-LEDs and G-LEDs, the difference between theintensity of divergent light and the intensity of light travelingstraight in the light guide plate is smaller than that of the R-LEDs andthe G-LEDs. In consequence, at the position at which light of one B-LEDstarts intersecting light of the adjacent B-LED, the difference betweenthe intensity of light traveling straight and the intensity of light atthe intersecting position becomes small, too, and the uniformity oflight power can be improved. Furthermore, the degree of colornon-uniformity becomes small when the color mixes with light of theR-LED and the G-LED.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which a plurality of R-LEDs, a plurality of G-LEDsand a plurality of B-LEDs are arranged, wherein a plurality of G-LEDsare arranged at one of the light incidence surfaces and the plurality ofR-LEDs and the plurality of B-LEDs are arranged at the opposing lightincidence surface.

According to this construction, a plurality of LEDs are allocated to thetwo opposing light incidence surfaces of the light guide plate and theheat generated by all the LEDs is divided into both sides of the lightguide plate. Therefore, heat radiation to the surroundings of the LEDsis facilitated and the LEDs can be cooled efficiently, so that servicelife of the LEDs can be extended. Further, only the G-LEDs whichgenerate heat most are arranged on one of the light incidence surfaces.Since only the G-LEDs having great heat generated are arranged on one ofthe light incidence surfaces, cooling can be conducted efficiently onboth sides of the light guide plate.

A side light type backlight, according to the invention, comprises alight source including a plurality of LEDs, and a light guide plate,opposing end surfaces of the light guide plate being light incidencesurfaces at which a plurality of R-LEDs, a plurality of G-LEDs and aplurality of B-LEDs are arranged, wherein the plurality of G-LEDs andthe plurality of B-LEDs are arranged at one of the light incidencesurfaces and the plurality of R-LEDs are arranged on the opposing lightincidence surface.

According to this construction, the LEDs are allocated to the two lightincidence surfaces of the light guide plate opposing each other and theheat generated by all the LEDs is divided into two sides of the lightguide plate. Therefore, heat radiation to the surroundings of the LEDsbecomes facilitated and the LEDs can be cooled efficiently, so thatservice life of the LEDs can be improved. Further, only the R-LEDseasily susceptible to the influences of the temperature are arranged onone of the light incidence surfaces. As only the R-LEDs are arranged onone of the light incidence surfaces, a cooling structure in which weightof cooling is considered more for the R-LED side becomes possible. Inother words, the R-LEDs, the light emission characteristics of which arelikely to fluctuate in accordance with the change of the temperature,are able to emit light more efficiently.

A side-light type backlight, according to the invention, comprises alight source including a plurality of LEDs, and a light guide plate, atleast one of opposing end surfaces of the light guide plate being lightincidence surface at which a plurality LEDs are arranged, wherein theplurality of LEDs include a plurality of white LEDs and a plurality ofB-LEDs.

According to this construction, the white chromaticity can be shiftedtowards the blue side by using the white LEDs and the B-LEDs incombination, in the case where the white chromaticity deviates towardsthe yellow side when only the white LEDs are used. Therefore, when thewhite chromaticity deviates from the specification value due to productvariance of the white LEDs, the white color can be adjusted to optimumwhite by adjusting the driving current of the B-LEDs.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, at least one of opposing end surfaces of the light guideplate being light incidence surface at which a plurality LEDs arearranged, wherein at least two rows of the plurality of LEDs arearranged in a direction of thickness of the light guide plate and theLEDs of each of the rows are arranged in a longitudinal direction of thelight incidence surface of the light guide plate.

According to this construction, a large number of LEDs can be arrangedeven when only one light incidence surface of the light guide plateexists, and high brightness can be achieved.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which a plurality LEDs are arranged, wherein theLEDs at both ends are arranged in such a fashion that a light emissionportion of each of the LEDs opposes an extreme end of the lightincidence surface of the light guide plate in a longitudinal direction.

According to this construction, because light can be sufficientlyirradiated onto edges of the light guide plate, the shadows of the edgesof the light guide plate become small and cannot be easily recognizedeven when they enter the display area. Therefore, the display qualitycan be maintained.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged wherein aframe is provided for accommodating the light source and the light guideplate, and a member having an elastic property or a spring property isarranged between the light source and the frame or between the lightincidence surface of the light guide plate and the opposing end surfacein such a fashion as to press the LEDs to the light guide plate.

According to this construction, the surfaces of the LEDs are broughtinto close contact with the light incidence surface of the light guideplate due to the urging operation of the member having the elasticproperty or the spring property. Therefore, light leaving the LEDs isefficiently made incident to the light guide plate and the utilizationefficiency of light can be improved.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinthe plurality of LEDs are mounted to a circuit substrate, and a memberhaving a higher heat conductivity than that of air is interposed betweenone of mounting surfaces and surfaces other than light outgoing surfacesof the LEDs and a surface of a housing of the light source or a surfaceof a frame constituting a backlight unit.

According to this construction, the LEDs and the housing aresubstantially maintained in close contact. Consequently, heat generatedfrom the LEDs satisfactorily flows to the housing and heat radiation ofthe LEDs can be made efficiently. Therefore, the LEDs can keep a highlight emission efficiency and a high brightness and the life can beimproved.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinthe plurality of LEDs are mounted on a circuit substrate, and one ofmounting surfaces and surfaces other than light outgoing surfaces of theLEDs keeps contact with a surface of a housing of the light sourceportion or a surface of a frame constituting a backlight unit.

According to this construction, the LEDs and the housing keepsubstantially the close contact state. Consequently, heat generated fromthe LEDs satisfactorily flows to the housing and heat radiation of theLEDs can be made efficiently. Therefore, the LED can keep a high lightemission efficiency and high luminance and the life can be improved.

A side light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinthe light guide plate has a taper shape at an end portion thereof on theside of the light incidence surface, and a jig for fixing the LEDs has ataper shape substantially coming into close contact with the taper shapeof the light guide plate.

According to this construction, the light guide plate having the taperportion and the light source including the LEDs can be stably fixed tothe jig. Further, a part of the light made incident to the light guideplate from the incidence surface of the light guide plate is reflectedby the tapered portion of the light guide plate and travels in the lightguide plate at an angle closer to the axis of the light guide plate.Therefore, the tapered portion of the light guide plate has the functionof narrowing the diverging angle of incident light to the light guideplate. Therefore, the amount of light of outgoing light from the lightguide plate can be increased.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinthe plurality of LEDs are arranged with intervals among them, and amember is interposed between the LED and the LED.

According to this construction, the LEDs can be fixed at predeterminedpositions, and the light emitting portions of the LEDs and the incidencesurface of the light guide plate can be brought more reliably intomutual contact.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinthe LEDs are mounted to a circuit substrate, and light outgoing surfacesof the LEDs protrude outward from the circuit substrate.

According to this construction, the outgoing surfaces of the LEDs canmore easily come into close contact with the incidence surface of thelight guide plate, and light can be therefore introduced efficiently.Therefore, a brighter backlight can be accomplished.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LED are arranged, wherein Mnumber of the LEDs are arranged in a longitudinal direction of the lightincidence surface of the light guide plate and electrically connectedsuch that there are a plurality of groups of LEDs with each groupcomprising N number of LEDs mutually adjacently arranged in series.

According to this construction, a driving voltage of the backlightcontaining a large umber of LEDs can be lowered.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereinM number of the LEDs are arranged in a longitudinal direction of thelight incidence surface of the light guide plate such that every Snumber of LEDs from the end are electrically connected to each other (S:a positive integer).

According to this construction, even when a certain LED becomesdefective (open state, for example), the LEDs with the interval of Sremain ON. Therefore, though the amount of light of the backlight drops,non-uniformity the amount of light cannot be recognized easily.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereina plurality of rows of the LEDs are arranged in a direction of thicknessof the light guide plate, with each row of the LEDs arranged in alongitudinal direction of the light incidence surface of the light guideplate and is electrically connected to one another in series.

According to this construction, even when any one of the LEDs becomesdefective, no influence is exerted on the LEDs of other stages.Therefore, non-uniformity of light power hardly occurs even though thelight power of the backlight changes.

A side-light type backlight, according to the present invention,comprises a light source including a plurality of LEDs, and a lightguide plate, opposing end surfaces of the light guide plate being lightincidence surfaces at which the plurality of LEDs are arranged, whereina plurality of rows of the LEDs are arranged in a direction of thicknessof the light guide plate, with each row of the LEDs arranged in alongitudinal direction of the light incidence surface of the light guideplate, N number of the LEDs being electrically connected in series inwhich the LEDs adjacent to each other are arranged in different rows.

According to this construction, non-uniformity the amount of light ofthe backlight can hardly be recognized even when any one of the LEDsbecomes defective. The LED driving voltage can be lowered and the powersource can be made compact in size.

A liquid crystal display device according to the invention includes thebacklight described above and a liquid crystal panel.

As explained above, the present invention can secure uniformity of theamount of light in the light guide plate and uniformity of chromaticity,can prevent the drop of light emission efficiency due to heat generatedby the LEDs, can cope with variance of the LEDs, can secure reliabilityand can improve efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription of the preferred embodiments, with reference to theaccompanying drawings, in which:

FIG. 1 is a view showing a liquid crystal display device;

FIG. 2 is a view showing a backlight according to an embodiment of thepresent invention including a light source including a plurality of LEDsand a light guide plate;

FIGS. 3A to 3C are views showing light distribution characteristics oflight emitted from LEDs of respective colors;

FIG. 4 is a view showing light distribution characteristics of lightoutgoing from the LEDs of respective colors in the light guide plate;

FIG. 5 is a schematic sectional view showing a typical example of theLED construction;

FIG. 6 is a plan view showing a circuit substrate having a plurality ofLEDs mounted thereto and a flexible circuit substrate connected to thecircuit substrate;

FIG. 7 is a sectional view showing an example of an LED housing foraccommodating a plurality of LEDs and a circuit substrate, and a lightguide plate;

FIG. 8 is a sectional view showing another example of an LED housing foraccommodating a plurality of LEDs and a circuit substrate, and a lightguide plate;

FIG. 9 is a view showing a backlight according to still anotherembodiment of the present invention;

FIG. 10 is a view showing a backlight according to a further embodimentof the invention;

FIG. 11 is a view showing a backlight according to a further embodimentof the invention;

FIG. 12 is a view showing a backlight according to a further embodimentof the invention;

FIG. 13 is a view showing the arrangement of the LEDs shown in FIG. 12;

FIG. 14 is a view showing a backlight according to a still furtherembodiment of the invention;

FIG. 15 is a view showing a backlight according to a still furtherembodiment of the invention;

FIG. 16 is a view showing the arrangement of the LEDs shown in FIG. 15;

FIG. 17 is a view showing a travel of light in the light guide plateshown in FIGS. 15 and 16;

FIG. 18 is a view showing a backlight according to a still furtherembodiment of the invention;

FIG. 19 is a view showing the arrangement of the LED shown in FIG. 18;

FIG. 20 is a view showing a backlight according to a further embodimentof the invention;

FIG. 21 is a sectional view of the backlight shown in FIG. 20;

FIG. 22 shows a backlight according to a further embodiment of theinvention;

FIG. 23 is a sectional view through the LED housing shown in FIG. 22;

FIG. 24 is a view showing a backlight according to a further embodimentof the invention;

FIG. 25 is a view showing a backlight according to a further embodimentof the invention;

FIG. 26 is a sectional view through the LED housing shown in FIG. 25;

FIG. 27 is a view showing a backlight according to a further embodimentof the invention;

FIG. 28 is a view showing a backlight according to a further embodimentof the invention;

FIG. 29 is a view showing a backlight according to a further embodimentof the invention;

FIG. 30 is a view showing the electric connection of the LEDs shown inFIG. 29;

FIG. 31 is a view showing the electric connection of the LEDs in anotherembodiment;

FIG. 32 is a view showing a backlight according to a further embodimentof the invention;

FIG. 33 is a view showing the electric connection of the LEDs shown inFIG. 32; and

FIG. 34 is a view showing the electric connection of the LEDs in afurther embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained with reference to thepreferred embodiments and by way of the drawings.

FIG. 1 shows a liquid crystal display device according to an embodimentof the present invention. FIG. 2 shows a backlight including a lightsource including a plurality of LEDs, and a light guide plate, accordingto the embodiment of the present invention.

In FIG. 1, the liquid crystal display device 10 includes a liquidcrystal panel 12 and a side light type backlight 14. The liquid crystalpanel 12 includes a color filter and, whenever necessary, polarizers.The backlight 14 irradiates the liquid crystal panel 12 with whitelight.

The backlight 14 includes a light source 18 including a plurality ofLEDs 16 and a light guide plate 20. The end surface 20A of the lightguide plate 20 is a light incidence surface at which a plurality of LEDs16 are arranged. The light guide plate 20 shown in FIG. 1 has a wedgeshape that is tapered from the end surface 20A as the light incidencesurface towards the opposite end surface. A reflection sheet 22 isarranged below the light guide plate 20 and optical sheets, such as adiffusion sheet 24 and a prism sheet 26, are arranged above the lightguide plate 20.

FIGS. 5 to 8 show a basic construction of the LED 16. FIG. 5 is aschematic sectional view showing a typical example of the constructionof the LED 16. The LED 16 is fabricated by molding a semiconductor chip30 mounted to a substrate 28 with a transparent resin 32. Electrodes 34are provided in the substrate 28 and the LED 16 is connected to a powersource through the electrodes 34. The LED 16 can emit light in adirection of an arrow A or an arrow B, for example. The size of thesemiconductor chip 30 is from 0.3 to 1 mm square, for example, and thesize of the LED 16 is from 0.5 to several mm square, for example.

FIG. 6 is a plan view showing a circuit substrate 36 having a pluralityof LEDs 16 mounted thereto and a flexible circuit substrate 38 connectedto the circuit substrate 36. The circuit substrate 36 has a lengthnecessary for mounting the plurality of LEDs 16 and the electrodes 34 ofthe LEDs 16 are connected to corresponding conductors of the circuitsubstrate 36. The flexible circuit substrate 38 is connected to thecircuit substrate 36 at the end of the circuit substrate 36 and isfurther connected to a power source and a control device, not shown inthe drawings. The circuit substrate 36 may be by itself a flexiblecircuit substrate.

FIG. 7 is a sectional view showing an example of an LED housing 40 foraccommodating a plurality of LED 16 and the circuit substrate 36, andthe light guide plate 20. The LED housing 40 is an elongated memberhaving a sectional shape of a horizontally inverted U. The length of theLED housing 40 corresponds to the length of the circuit substrate 36 inFIG. 6, for example. An opening on one of the sides of the LED housing40 faces the light guide plate 20. The circuit substrate 36 to which aplurality of LED 16 is mounted under the state shown in FIG. 6 isaccommodated in the LED housing 40. In FIG. 7, the circuit substrate 36is fixed to the bottom wall of the LED housing 40 through an adhesive,for example. The LED 16 emits light towards the light guide plate 20 asindicated by the arrow B, for example.

FIG. 8 is a sectional view showing an example of the LED housing 40accommodating a plurality of LEDs 16 and the circuit substrate 36, andthe light guide plate 20. In this case, the circuit substrate 36 isfixed to a vertical wall of the LED housing 40. The LED 16 emits lighttowards the light guide plate 20 as indicated by the arrow A, forexample.

FIG. 1 shows only the LED 16 and the LED housing 40 as the light source18.

In FIG. 2, a plurality of LEDs 16 include a plurality of R-LEDs (redLEDs), a plurality of G-LEDs (green LEDs) and a plurality of B-LEDs(blue LEDs). Marks R, G and B are allocated to the LEDs 16,respectively. The number of R-LEDs 16, the number of G-LEDs 16 and thenumber of B-LEDs 16 are different from one another. This embodiment isdirected to appropriately constitute a white light source by all theLEDs 16. In this case, the amount of light of G-LEDs 16 is preferablygreatest while the amount of light of B-LEDs 16 is preferably smallest.Therefore, if the amounts of light of all the LEDs 16 are the same, thenumber of G-LEDs 16 is the greatest and the number of B-LEDs 16 is thesmallest. However, the number of the LEDs 16 of respective colors is notlimited to the proportion shown in the drawing.

FIGS. 3A to 3C show light distribution characteristics emitted from theLEDs 16 of respective colors. FIG. 3A shows the light distributioncharacteristics of the R-LEDs 16. FIG. 3B shows the light distributioncharacteristics of the G-LEDs 16. FIG. 3C shows the light distributioncharacteristics of the B-LEDs 16. The light distribution range (theangular range in which light diverges) is smallest in the lightdistribution characteristics of the G-LEDs 16 and is greatest in thelight distribution characteristics of the B-LEDs 16. In other words, therelationship of: the light distribution range of G-LEDs 16<the lightdistribution range of R-LEDs 16<the light distribution range of B-LEDs16 is satisfied.

FIG. 4 shows distribution characteristics of light leaving the LEDs ofrespective colors in the light guide plate. In the light guide plate 20,light leaving the B-LEDs 16 and traveling at a relatively large angle inthe light guide plate 20 has a relatively high brightness and lightleaving the G-LEDs 16 and traveling at a relatively small angle hasrelatively low brightness.

In this embodiment, the light guide plate 20 is made of an acrylicresin. Light incident, from each LED 1, onto the light guide plate 20travels with a divergent range of about ±42°. As to the distributioncharacteristics of each color, the light distribution characteristics ofthe B-LEDs 16 has the highest uniformity and the light distributioncharacteristics of the G-LEDs 16 has peak distribution characteristicsas shown in FIG. 3. Therefore, in the distribution characteristics oflight made incident into the light guide plate 20, light exists withinthe range of ±42° as shown in FIG. 4, and B (blue) light has higherintensity at 42° than R (red) and G (green) light at ±42°. As a result,although the interval between the B-LEDs 16 is greater than the intervalbetween the LEDs 16 of the other color, the light mixed in the lightguide plate 20 is apt to become uniform. As to the R-LEDs 16 and theG-LEDs 16, on the other hand, even when light of R (red) or light of G(green) starts mix, non-uniformity of the amount of light occurs at aposition in the proximity of the end surface 20A at which the colormixture starts occurring because the difference of the light intensityis small between the 0° direction and the 42° direction. As a result,even in the construction in which the number of the B-LEDs 16 is smalland the LEDs are arranged at greater intervals, the light intensity canbe made uniform at a position near the light incidence surface 20A ofthe light guide plate 20. Also, a color non-uniformity resulting fromcolor mixing can be reduced.

The number of the LEDs 16 and the arrangement positions are not limitedto those shown in the drawings and they are the parameters that aredesigned in accordance with the characteristics of the LEDs 16 to beused and the specification of chromaticity.

FIG. 9 shows a backlight according to another embodiment of the presentinvention. This embodiment has substantially the same basic constructionas that of the foregoing embodiment. The backlight 14 has a light source18 including a plurality of LEDs 16, and a light guide plate 20. In theembodiments explained below, too, the backlight includes the lightsource including a plurality of LEDs 16, and the light guide plate 20,as described above. Therefore, a repeated explanation of the basicconstruction of the following embodiments will be omitted.

In this embodiment, opposing end surfaces 20A and 20B of the light guideplate 20 are light incidence surfaces and the LEDs 16 of R, G and B areassigned to the light incidence surfaces 20A and 20B. A plurality ofG-LEDs 16 are arranged at one of the light incidence surfaces 20A and aplurality of R-LEDs 16 and a plurality of B-LEDs 16 are arranged at theopposing light incidence surface 20B.

When the white balance is secured, the number of G-LEDs 16 becomes thegreatest as described above, and therefore, only the G-LEDs 16 arearranged at one of the light incidence surfaces 20A. In consequence, theamount of heat generated by all the LEDs 16 is divided substantiallyuniformly between the light incidence surface 20A and the lightincidence surface 20B. Therefore, a large number of LEDs 16 can becooled efficiently, the light emission efficiency is not reduced and theservice life can be prolonged. In this case, the LEDs 16 may have thelight distribution characteristics shown in FIGS. 3 and 4.

FIG. 10 shows a backlight according to a further embodiment of thepresent invention. In this embodiment, the opposing end surfaces 20A and20B of the light guide plate 20 are the light incidence surfaces and theR, G and B-LEDs 16 are assigned to the light incidence surfaces 20A and20B.

A plurality of G-LEDs 16 and a plurality of B-LEDs 16 are arranged onthe side of one of the light incidence surfaces 20A, and a plurality ofR-LEDs 16 are arranged on the side of the opposing light incidencesurface 20B. Because the R-LEDs 16 are most susceptible to thetemperature, only the R-LEDs 16 are arranged on the side of one of thelight incidence surfaces 20B. In consequence, the R-LEDs 16 can beefficiently cooled, light emission efficiency of the R-LEDs 16, whichare most susceptible to the temperature, can be improved and the servicelife can be prolonged.

FIG. 11 shows a backlight according to a further embodiment of thepresent invention. In this embodiment, one of the end surfaces 20A ofthe light guide plate 20 is the light incidence surface. A plurality ofwhite LEDs 16 and a plurality of B-LEDs 16 are arranged at the lightincidence surface 20A. The white LEDs 16 are represented by symbol W.

In the construction in which the white LEDs 16 and B-LEDs 16 are used incombination, the B-LEDs 16 can shift the white chromaticity towards theblue side to accomplish a desired white chromaticity because, when onlythe white LEDs 16 are used, the white chromaticity is closer to theyellow side. Therefore, when white chromaticity deviates from thespecification value due to variance of the products of the white LEDs16, the white color can be adjusted to the optimum white color byadjusting a driving current of the B-LEDs 16. Though this embodimentuses the B-LEDs 16, the G-LEDs or the R-LEDs may well be used, as well.

One of the end surfaces 20A of the light guide plate 20 is the lightincidence surface in the illustrated embodiment, but both end surfaces20A and 20B of the light guide plate 20 may be used as the lightincidence surfaces. In this case, a plurality of white LEDs 16 and aplurality of B-LEDs 16 are arranged on each of the light incidencesurfaces 20A and 20B. It is also possible to arrange the white LEDs 16on one of the light incidence surfaces and the B-LEDs 16 on the opposinglight incidence surface.

FIG. 12 shows a backlight according to a still further embodiment of thepresent invention. FIG. 13 shows the arrangement of the LEDs shown inFIG. 12. In this embodiment, the LEDs 16 are arranged in upper and lowerrows at the light incidence surface 20A of the light guide plate 20. Asshown in FIG. 13, the LEDs 16 of the upper and lower rows are arrangedin such a fashion as to fully cover the light incidence surface 20A ofthe light guide plate 20.

In this case, the LED housing 40 is formed by two L-shaped metal plates40A and 40B bonded together by an adhesive sheet 42. The LEDs 16 of thelower row are fixed to the bottom wall of the L-shaped metal plate 40Athrough the circuit substrate 36 and the LEDs 16 of the upper row arefixed to the upper wall of the L-shaped metal plate 40B through thecircuit substrate 36. The LEDs 16 are of the side view type that emitslight in the direction of the arrow B in FIG. 5. The circuit substrate36 may be a flexible circuit substrate. The thickness of the light guideplate 20 on the side of the light incidence surface is 2 mm and thethickness of the LEDs 16 is 0.8 mm, by way of example. Therefore, theLEDs 16 can be arranged in the upper and lower rows. Because a largenumber of LEDs 16 can be arranged even in the case of the wedge-shapedlight guide plate 20 having only one light incidence surface, highbrightness can be accomplished.

FIG. 14 shows a backlight according to a still further embodiment of thepresent invention. In the embodiment shown in FIG. 14, the LEDs 16 arearranged at the light incidence surface 20A of the light guide plate 20in two, upper and lower, rows, in the same way as in the embodimentsshown in FIGS. 12 and 13. The LEDs 16 of the upper and lower rows arearranged in two rows on a single circuit substrate (or a flexiblecircuit substrate) 36. The circuit substrate 36 is fixed to the verticalwall of the LED housing 40 through a metal plate 44 of aluminum, or thelike. The metal plate 44 assists heat radiation of the LEDs 16. The LEDs16 shown in FIG. 14 are of a top view type that emits light in thedirection indicated by the arrow A in FIG. 5. FIGS. 12 to 14 show theLEDs 16 having the two-row construction but the LEDs 16 may beconstituted into three or more rows.

FIG. 15 shows a backlight according to a still further embodiment of thepresent invention. FIG. 16 shows the arrangement of the LEDs shown inFIG. 15. The LEDs 16 are arranged at the light incidence surface 20A ofthe light guide plate 20 in the upper and lower rows in the same way asin the embodiments shown in FIGS. 12 to 14. In FIG. 15, the LEDs 16 ofthe lower row are arranged between two adjacent LEDs 16 of the upperrow. In other words, the LEDs 16 in the two rows are arranged in astaggered arrangement.

FIG. 17 shows the path of light in the light guide plate shown in FIGS.15 and 16. The rays of light entering the light guide plate 20 from theLEDs 16 of the upper row are indicated by solid lines. The rays of lightentering the light guide plate 20 from the LEDs 16 of the lower row areindicated by broken lines. When the LEDs 16 are viewed from above thelight guide plate 16, the gap between the LED 16 and another LED 16 is½, compared with that in one row, and accordingly, light mixes well at aposition close to the light incidence surface 20A of the light guideplate 20 and non-uniformity of the amount of light becomes small.Consequently, a region in which the rays of light coming from the LEDs16 of the lower row exist is positioned at a region in which the rays oflight of the LEDs 16 of the upper row incident to the light guide plate20 do not exist, and non-uniformity of the amount of light in theproximity of the light incidence surface of the light guide plate 20 canbe improved.

FIG. 18 shows a backlight according to a still further embodiment of thepresent invention. FIG. 19 shows the arrangement of the LEDs shown inFIG. 18. The LEDs 16 at both ends are arranged in such a fashion thatthe light emission portion 20L of the LED 19 opposes the extreme ends(both edges) of the light incidence surface 20A of the light guide plate20. In the prior art devices, because light from the light source doesnot exist at the edges of the light guide plate or the amount of lightis lower than that at other portions, the edges of the light guide platehave a shadow and the display quality drops. According to theconstruction of this embodiment, however, light having sufficient lightintensity is irradiated to the edges of the light guide plate, theshadow at the edges of the light guide plate becomes small and theshadow of the edges cannot be recognized easily even when it enters thedisplay area. In consequence, display quality can be maintained.

FIG. 20 shows a backlight according to a still further embodiment of thepresent invention. FIG. 21 is a sectional view of the backlight shown inFIG. 20. A plurality of LEDs 16 are mounted in a row on the circuitsubstrate 36 (not shown in FIGS. 20 and 21) and arranged in the LEDhousing 40. The backlight comprising the light source including the LEDs16 and the light guide plate is accommodated in a plastic frame 46. Ametal frame can be also used in place of the plastic frame 46.

The edge portion of the light guide plate 20 having the light incidencesurface 20A is inserted and fitted in the opening of the LED housing 40so that the surfaces of the LEDs 16 are brought into close contact withthe light incidence surface 20A of the light guide plate 20. Inaddition, a buffer material (such as a rubber plate or a gel-like plate)48 is inserted between the inner surface of the plastic frame 46 and theouter surface of the LED housing 40. The buffer material 48 presses theLED housing 40 towards the light guide plate 20 with a certain springforce. As a result, the surfaces of the LEDs 16 inside the LED housing40 and the incidence surface 20A of the light guide plate 20 aremaintained in close contact. Therefore, light outgoing from the LEDs 16efficiently enters the light guide plate 20 and utilization efficiencyof light can be improved. Even when the light guide plate 20 undergoesexpansion and contraction due to the change in the environment, thebuffer material 48 correspondingly functions to keep contact between theLEDs 16 in the LED housing 40 and the light guide plate 20 withouthindering expansion and contraction of the light guide plat 20.

Also, the opposing end surfaces of the light guide plate 20 are pressedto the inner surfaces of the plastic frame 46. When the plastic frame 46is formed of a white color material having low light absorption, lightleaking from the opposing end surfaces of the light guide plate 20 isreflected by the inner surfaces of the plastic frame 46, returns againinto the light guide plate 20 and is utilized again. Otherwise, asimilar effect can be obtained by inserting a reflecting sheet betweenthe opposing end surfaces of the light guide plate 20 and the innersurfaces of the plastic frame 46.

FIG. 22 shows a backlight according to a still further embodiment of thepresent invention. FIG. 23 is a sectional view through the LED housingshown in FIG. 20. A plurality of LEDs 16 are arranged in a row in theLED housing 40 through the circuit substrate 36. The circuit substrate36 is fixed to the bottom wall of the LED housing 40. A soft sheet-likemember 50 having a high heat conductivity is inserted between the upperwall of the LED housing 40 and the LEDs 16 so that the LEDs 16 and theupper wall of the LED housing 40 are in substantially close contactthrough the sheet-like member 50. Consequently, the heat generated bythe LEDs 16 can flow to the upper and lower surfaces and the heat of theLEDs 16 can be efficiently dissipated. In consequence, light emissionefficiency of the LEDs 16 can be maintained and high brightness and longservice life can be provided. Even when the soft sheet-like member 50having a high heat conductivity described above is not used, the heatradiation effect can be acquired when a member having high heatradiation property (resin type sheet, resin type adhesive sheet, etc) isused.

In the embodiment described above, it is also possible to use a housingconstruction in which a gap between the upper surfaces of the LEDs 16opposite to the mounting surfaces of the LEDs 16 and the inner surfaceof the LED housing 40 is eliminated and the inner surface of the LEDhousing 40 keeps close contact with the LEDs 16. In the backlightaccording to the prior art, an air layer having a certain thicknessexists at this portion and heat radiation performance is low. When thisair layer is eliminated, the heat of the LEDs 16 can be quicklytransferred to the LED housing 40 and is radiated outside from the LEDhousing 40.

FIG. 24 shows a backlight according to a still further embodiment of thepresent invention. The plastic frame (or metal frame) 46 is furtheraccommodated in a bezel 54. The end portion of the light guide plate 20on the side of the light incidence surface is a tapered portion 20T withthe taper provided in the direction of thickness. The LEDs 16 keep closecontact with a narrow end surface (light incidence surface) at thedistal end of the tapered portion 20T. The LED housing 40, as a jig forfixing the LEDs, includes an upper support frame 40U and a lower supportframe 40L. The upper and lower support frames 40U and 40L have taperedportions 40UT and 40LT corresponding to the taper of the light guideplate 20. The lower support frame 40L has a seat portion 40S continuingthe tapered portion 40LT for supporting the LEDs 16 fitted to thecircuit substrate 36.

The upper support frame 40U and the lower support frame 40L are formedof a white color material free from light absorption or their surfacesare made of a member having a high reflection factor. Therefore, lightleaking out from the surface other than the light incidence surface ofthe light guide plate 20 is again returned into the light guide plate 20and light utilization ratio can be therefore improved.

In this construction, the tapered portion 20T of the light guide plate20 is brought into close contact with the tapered portions 40UT and 40LTof the upper and lower support frames 40U and 40L and are fixed in sucha condition that the light emission surfaces of the LEDs 16 abut againstthe light incidence surface of the light guide plate 20 without beingdeviated. According to this construction, the light source including thelight guide plate 20 having the tapered portion 20T, and the LEDs 16,can be stably fixed to the LED housing 40. Because the light guide plate20 has the tapered portion 20T, a part of light made incident from theincidence surface of the light guide plate 20 into the light guide plate20 is reflected by the tapered portion 20T of the light guide plate 20and travels in the light guide plate 20 at an angle approaching the axisof the light guide plate 20. Therefore, the tapered portion 20T has afunction of reducing the expansion angle of incident angle of the lightguide plate 20 and the amount of light of outgoing light from the lightguide plate 20 can be improved.

FIG. 25 shows a backlight according to a still further embodiment of thepresent invention and is a sectional view taken along a line XXV-XXV ofFIG. 26. The light guide plate 20 is omitted from FIG. 26. In thisembodiment, the light guide plate 20 has the tapered portion 20T, theupper and lower support frames 40U and 40L of the LED housing 40 havethe tapered portions 40UT and 40LT, and the lower support frame 40L hasthe seat portion 40D, in the same way as in the embodiment shown inFIGS. 23 and 24. The upper and lower support frames 40U and 40L areformed of a white color material free from light absorption or the tapersurface is a reflection surface.

A plurality of LEDs 16 are spaced apart from each other, and a member 56having a reflection function is interposed between the LEDs 16. In thisembodiment, the upper support frame 40U is shaped into a shape havingprojections and recesses, and the LEDs 16 are arranged in the recesses,while the projections forming the member 56 having the reflectionfunction. In this case, the upper support frame 40U and the lowersupport frame 40L are formed of a white color material free from lightabsorption, or their surfaces are members having a high reflectionfactor. Therefore, this embodiment has an effect similar to the functionand effect of the embodiment shown in FIGS. 23 and 24 and the LEDs 16are fixed at the predetermined position, so that the light emissionportions of the LEDs 16 and the incidence surface of the light guideplate 20 can be brought more reliably into contact with each other.Therefore, a greater amount of light enters the light guide plate 20from the LEDs 16. However, this embodiment can be applied also to thelight guide plate not having the taper shape so long as a plurality ofLEDs 16 are arranged with the gap and the member 56 is interposedbetween the LED 16 and the LED 16.

FIG. 27 shows a backlight according to a still further embodiment of thepresent invention. This embodiment is constituted in a way similar tothe embodiment shown in FIGS. 23 and 24 except that it is so constitutedas to match the LEDs 16 arranged in two rows. The light guide plate 20in this embodiment has a portion 20L coming into contact with the LEDs16 of the lower row and a portion 20U coming into contact with the LEDs16 of the upper row. Further, the LED housing 40 includes the uppersupport frame 40U, the lower support frame 40L and an intermediatesupport frame 40C. Each part of the light guide plate 20 and the LEDhousing 40 includes a tapered portion similar to the tapered portionrespectively shown in FIGS. 23 and 24. The intermediate support frame40C is inserted between the upper support frame 40U and the lowersupport frame 40L and between the portions 20L and 20U of the lightguide plate 20.

Therefore, the embodiment shown in FIG. 27 provides the function andeffect similar to those of the embodiments shown in FIGS. 23 to 26.

FIG. 28 shows a backlight according to a still further embodiment of thepresent invention. In this embodiment, the LEDs 16 are mounted in such afashion that the light outgoing surfaces protrude forward, by the gap G,from the circuit substrate 36. The light guide plate 20 enters the LEDhousing 40. Accordingly, the light outgoing surface of the LED 16 can bemore easily brought into close contact with the incidence surface of thelight guide plate 20 and, hence, light can be introduced moreefficiently into the light guide plate 20. Consequently, a brighterbacklight can be provided.

FIG. 29 shows a backlight according to a still further embodiment of thepresent invention. FIG. 30 shows an electric connection of the LEDs inFIG. 29. A plurality of LEDs 16 are arranged in a row at the lightincidence surface of the light guide plate 20. The first to seventh LEDs16 from the end of the row of the LEDs are connected in series as oneLED group and the eighth to fourteenth LEDs are connected in series as anext LED group. Subsequent groups of the seven pieces of LEDs 16 areconnected to one after another in series. The groups each containing theseven pieces of LEDs 6 connected in series are connected in parallel.

Therefore, when a driving voltage of one LED 16 is 3.5 V, the drivingvoltage of the entire LEDs 16 is 24.5 V. Assuming that the number of theLEDs 16 is 49 and all the LEDs 16 are connected in series, the drivingvoltage is 171.5 V. In this case, the driving power source unavoidablybecomes a high voltage circuit and sizes and constructions of componentsbecome great. According to the construction of this embodiment, however,the driving voltage becomes lower and the power source can be easilyrendered compact. The merit of this embodiment is not limited to thedriving power source. Assuming that any one of the LEDs 16 becomesdefective, only the group in which that LED 16 is connected in seriesbecomes defective and the influence on the backlight can be reduced.

In the backlight according to this embodiment, M number of LEDs arearranged in the longitudinal direction of the light incidence surface ofthe light guide plate 20 and electrically connected so that there are aplurality of groups of LEDs with each group comprising N number of LEDsmutually adjacently arranged in series.

In this embodiment, seven LEDs 16 are connected in series as one groupbut this arrangement is not restrictive. A power source may be disposedindependently in the unit of seven LEDs. The number of LEDs 16 in onegroup is preferably from 2 to 10.

FIG. 31 shows another electric connection of the LEDs. A plurality ofLEDs 16 are arranged in a row as shown in FIG. 30. In FIG. 31, everyother LED 16 is connected in series. In other words, the first, third,fifth and seventh LEDs 15 are connected in series, and the second,fourth and sixth LEDs 16 are connected in series. According to thiscircuit construction, even when a certain LED 16 becomes defective(open, for example), every other LED 16 is kept ON. Therefore, eventhough light power of the backlight drops to a half, non-uniformity oflight power cannot be easily recognized.

In the drawing, every other LED 16 is shown connected in series, but anarbitrary number of LEDs may exist between the two LEDs connectedelectrically. In other words, in the backlight of this embodiment, Mnumber of LEDs are arranged in the longitudinal direction of the lightincidence surface of the light guide plate such that every S number ofthe LEDs from the end are electrically connected to each other (S: apositive integer). In this case, S is preferably from 1 to 10.

FIG. 32 shows a backlight according to a still further embodiment of thepresent invention. FIG. 33 shows electric connection of the LEDs shownin FIG. 32. In this embodiment, a plurality of LEDs 16 are arrangedzigzag in two rows with respect to the thickness direction of the lightincidence surface of the light guide plate 20 in such a fashion that theLEDs 16 of the lower row are positioned between the two LEDs 16 of theupper row. Odd numbers (1, 3, 5) are allocated to the LEDs 16 of theupper row and even numbers (2, 4, 6) are allocated to the LEDs 16 of thelower row. The LEDs 16 of each row are arranged in series. Therefore,even when any one of the LEDs 16 becomes defective, no influences aregiven on the LEDs 16 of the other row. Consequently, though light powerof the backlight changes, non-uniformity of light power hardly occurs.

FIG. 34 shows another electric connection of the LEDs. A plurality ofLEDs 16 are arranged in two staggered rows in the thickness direction ofthe light incidence surface of the light guide plate 20. Odd numbers (1,3, 5) are allocated to the LEDs 16 of the upper row and even numbers (2,4, 6) are allocated to the LEDs 16 of the lower row.

The LEDs 16 of the upper row and the LEDs 16 of the lower row arealternately connected to each other and moreover, with an interval offive LEDs 16. In other words, the first LED 16 and the sixth LED 16 areconnected in series and the eleventh LED 16 and the sixteenth LED 16 areconnected in series. This also holds true of other LEDs 16.Consequently, even when any one of the LEDs 16 becomes defective,non-uniformity of light power of the backlight can be hardly recognized.The LED driving voltage can be lowered and the power source can be madecompact in size.

In the backlight according to this embodiment, a plurality of rows ofthe LEDs are arranged in the direction of a thickness of the light guideplate with each row of LEDs arranged in the longitudinal direction ofthe light incidence surface of the light guide plate, N number of LEDsbeing electrically connected LED in series in which the LED adjacenteach other belong to different rows.

In all the embodiments explained above, the backlight can include atleast one of the reflection sheet, the diffusion sheet and the lenssheet. Furthermore, as shown in FIG. 1, for example, a liquid crystaldisplay device can be constituted by combining any one of the abovedescribed the backlights with a liquid crystal panel.

1. A side-light type backlight comprising: a light source including aplurality of LEDs, and a light guide plate, opposing end surfaces of thelight guide plate being light incidence surfaces at one of which theplurality of LEDs are arranged, wherein M of the LEDs are arranged in alongitudinal direction of the light incidence surface of the light guideplate and electrically connected such that there are a plurality ofgroups of the LEDs with each group comprising N LEDs mutually adjacentlyarranged in series, wherein said plurality of groups are adjacentlyarranged in a longitudinal line, and further wherein at least one ofsaid groups of N adjacent LEDs is a separate parallel connection from anadjacent group of N LEDs, wherein the plurality of LEDs includes LEDshaving a plurality of colors, and further wherein the LEDs arranged inseries include LEDs having colors different from one another.
 2. Theside-light type backlight as defined in claim 1, wherein N is an integerbetween 2 and
 10. 3. The side-light type backlight as defined in claim1, wherein said light guide plate is tapered from the light incidencesurface towards an opposite end surface.
 4. The side-light typebacklight as defined in claim 3, wherein the light incidence surface isbroader than the opposite end surface.
 5. The side-light type backlightas defined in claim 1, wherein said plurality of groups of LEDs areadjacently arranged in a single longitudinal line.
 6. The side-lighttype backlight as set forth in claim 1, wherein: the LEDs arranged inseries are set to emit light beams such that white light is obtainedwhen the light beams are mixed with one another.
 7. The side-light typebacklight as set forth in claim 6, wherein: the plurality of LEDs areset to have an identical amount of light, and the respective numbers ofLEDs are different for the colors.
 8. The side-light type backlight asset forth in claim 7, wherein: the plurality of LEDs include red LEDs,blue LEDs, and green LEDs, and the numbers of the red LEDs, the blueLEDs, and the green LEDs are different from one another such that thenumber of the blue LEDs is smaller than the number of the red LEDs, andthe number of the red LEDs is smaller than the number of the green LEDs.